Since Dr. Duwez invented Splat Cooling process in 1960, rapid solidification technique has become a newly developed field in metallurgical engineering and a new route for promoting mechanical and physical properties of different kinds of alloys. The rapid solidification technique has the merits in refining of microstructure, extension of solubility limit, producing homogeneous concentration distribution, and formation of amorphous phase. Such characteristics give more freedom to alloy design, and achieve better mechanical and physical properties than conventional processes can ever do.
The purpose of rapidly-solidified powder metallurgy is to solidify molten metal at a high cooling rate (higher than 10.sup.2 K/sec) to form grain-like, flake-like, or strip-like metal particles. The metal particles are then pressed, sintered, and hot worked to produce final products. According to relevant researches, the quality of the rapidly-solidified powder has a great effect on the mechanical properties of the alloy products. Therefore, the manufacturing process of the rapidly-solidified powder is the very decisive step in the whole-process.
Many methods of producing rapidly-solidified powder have been developed. Basically, for achieving the highest cooling rate, each method needs to cause at least one dimension of the powder products as small as possible, so as to transfer heat to a cooling medium as soon as possible.
For example, the air atomization method utilizes air atomizing and air cooling. The powder product is sphere-like, with a cooling rate at about 10.sup.2-3 K/sec. Since the kinetic energy is mostly used in accelerating the molten metal, the energy used in atomizing is quite low, only about 2% to 4%.
Another example is the Alcoa Spray method developed by American Aluminum company (ALCOA), which uses air atomization and metal substrate cooling. Molten metal is atomized by air, and then sprayed upon the surface of a water cooled roller which rotates rapidly. Metal flakes attached on the roller are stripped off by brushes and gathered into a collector. The cooling rate is up to about 10.sup.5 K/sec. The flakes are disk-like, but often unflatten and overlapped. Similarly, the momentum transfer efficiency of air atomization used in this method is low.
Rapid Solidification Rate (RSR) technology developed by Pratt & Whitney Co., U.S. Pat. No. 4,078,873 and U.S. Pat. No. 4,343,750 represent utilize centrifugal atomization and helium cooling. Molten metal flows through a funnel onto a disk rotating at a high speed (about 24000 rpm). By centrifugal force, it is accelerated radially, and then atomized into drops after leaving the disk. The droplets are rapidly cooled by circulating helium atmosphere, and solidify to form sphere-like powder. The atomization efficiency of RSR is relatively high. However, the cooling rate of RSR is about 10.sup.5 K/sec. The RSR is expensive because it uses helium cooling.